1. Field of the Invention
This invention relates to a combustion catalyst suitable for such a combustion device as the gas turbine which is required to possess durability to resist elevated temperatures.
2. Description of the Related Art
In recent years, the universal appreciation of the eventual exhaustion of the earth's oil resources has become to direct due attention to the necessity for developing an alternative energy and to the imperativeness of efficient utilization of energy resources. To fulfill the necessity, a gas turbine.cndot.steam turbine combined cycle electric power generation system using natural gas as the fuel and a coal gasification gas turbine.cndot.steam turbine combined cycle electric power generation system have been developed. Since these electric power generation systems enjoy high efficiency of power generation as compared with the electric power generation systems operated with the conventional steam turbines using fossil fuels, they are attracting attention as systems which are capable of effectively converting such fuels as natural gas and coal gas into electric power.
Incidentally, for the gas turbine combustor which is used in the gas turbine electric power generation system, the method of combustion by a homogeneous system (vapor-phase) reaction which implements the combustion of a gas containing a fuel gas and an oxidizing gas (gas for combustion) such as, for example, a mixture of a fuel gas with air by igniting the mixture by means of a spark plug is adopted.
FIG. 6 is a cross section illustrating an example of the construction of the essential part of a gas turbine combustor; 1 standing for a housing, 2 for a combustion nozzle, 3 for a spark plug (ignition element), and 4 for an a gas feed path provided on a lateral wall thereof with an air feed inlet 4a for feeding air as one of the components for combustion, a cooling air feed inlet 4b, and a diluting air feed inlet 4c and adapted to feed a required combustion gas to a turbine nozzle 5. In the combustor mentioned above, the fuel gas spouted through the combustion nozzle 2 is mixed with the air fed through the combustion air feed inlet 4a and the resultant mixture is ignited for combustion by the spark plug 3. In consequence of this combustion, necessary air supplies are made through the cooling air feed inlet 4b and the diluting air feed inlet 4c and the combustion gas which has been consequently cooled to a prescribed temperature (the turbine inlet temperature) is advanced through the turbine nozzle 5 and injected into the turbine.
Since the gas turbine combustor mentioned above generally uses air as the gas for combustion, however, the formation of nitrogen oxides (NO.sub.x) during the combustion poses a problem. The amount of nitrogen oxides so formed abruptly increases when the combustion temperature surpasses 1500.degree. C. Since a distribution of the fuel concentration is present inside the combustor, the interior of the combustor partly contains sites of temperatures exceeding 1500.degree. C. Thus, the gas turbine inevitably entails formation of nitrogen oxides in a large amount and necessitates extra provision of an expensive SCR (Selective Catalytic Reduction).
To cope with this problem, the method of combustion which comprises causing a heterogeneous system reaction by burning a combustion gas by means of a catalyst and further continuously causing a vapor-phase combustion has been proposed (Japanese Patent Publication No. HEI-02-45,772). Since this method of combustion resorting to a catalyst permits the combustion to start at a relatively low temperature and causes the temperature of this combustion to rise gently and enables the maximum temperature of the combustion to be repressed to a low level, it is at an advantage in not merely enabling the combustor itself to enjoy durability but also decreasing the amount of nitrogen oxides to be formed even when air is adopted as the oxidizing gas in the combustion gas (the gas comprising a fuel gas and an oxidizing gas).
FIG. 7 is a cross section illustrating with a model part of the construction of a catalyst used in a gas turbine combustor of the conventional method of catalytic combustion mentioned above; 6 standing for a durable support provided with a multiplicity of independent partitioned fuel gas flow paths 6a and 7 for an active catalyst deposited in the form of a coating on the inner wall surfaces of the combustion gas flow paths 6a of the durable support 6 mentioned above.
Incidentally, as a concrete example of the active catalyst, an active catalyst which has palladium and/or palladium oxide as main components thereof may be cited.
The active catalyst having palladium and/or palladium oxide as the main components thereof as mentioned above, however, has the state thereof varied by the oxygen release equilibrium which is fixed by the oxygen partial pressure and the temperature of the ambient air as illustrated in FIG. 8. Specifically, the palladium in the active catalyst is liable to assume the form of metallic palladium when the temperature is higher than the equilibrium temperature and the form of palladium oxide when the temperature is lower than the equilibrium temperature. At temperatures exceeding 900.degree. C., for example, substantially all the palladium assumes the state of metallic palladium even when the partial pressure of oxygen is about 1 atmosphere. Since the metallic palladium has a lower catalytic activity than palladium oxide, the catalytic activity is lowered and the heat quantity generated in consequence of combustion is decreased and the catalyst temperature is made to level off when the temperature is higher than the temperature of the oxygen release equilibrium. As a result, the active catalyst acquires the self control property of preventing its own temperature from rising above a fixed level and, consequently, adapts itself for a gas turbine.
Where other noble metal such as, for example, platinum is used for a catalyst, the activity of this catalyst continues to increase with the elevation of the temperature of the catalyst. As a result, this catalyst is at a disadvantage in entailing the so-called temperature run-away that the catalytic activity is exalted and the temperature of the catalyst is further elevated even by only a slight rise of temperature due to the temperature of the combustion gas and the concentration of the fuel.
Though the palladium based catalyst to be used in the method of combustion mentioned above has the advantage of permitting easy repression of the temperature run-away, it still has a problem in terms of service in the actual operation of a gas turbine.
When a combustion catalyst of palladium measuring 17 cm in length and having a construction the essential part of which is illustrated in a cross section in FIG. 7 is tested with the fuel concentration varied in the combustion gas fed to the inlet of the combustion gas flow path 6a to evaluate the combustion catalyst based on the efficiency of the palladium based catalyst and the temperature of the combustion gas at the outlet of the combustion gas flow path 6a, the results are as shown in FIG. 9. First, it is remarked that while the fuel concentration is low at the inlet of the combustion gas flow path 6a, the amount of the fuel consumed by the reaction per unit time per unit amount of catalyst is increased and the efficiency of the catalyst is exalted in proportion as the concentration is increased. In FIG. 9, the curve 1 represents the efficiency of the catalyst and the curve 2 the temperature of the catalyst. The term "efficiency of catalyst" as used herein means the ratio of the amount of the fuel burned to the total amount of the fuel supplied.
When the fuel concentration is further increased and, as a consequence, the temperature of the catalyst elevated by the combustion is caused to surpass the oxygen release temperature of palladium oxide, however, there arises an area in which the activity of the catalyst levels off. In consequence of this maximization of the activity of the catalyst, the temperature of the combustion gas at the outlet of the combustion gas flow path 6a levels off in the neighborhood of the equilibrium oxygen release equilibrium temperature of palladium oxide.
When the concentration of the fuel in the combustion gas is further increased, there arises an area in which a vapor-phase homogeneous reaction (combustion) abruptly occurs in the combustion catalyst (as in the combustion gas flow path 6a, for example) and the efficiency of the catalyst and the temperature of the catalyst are increased. In this area, practical operation of the combustion catalyst is no longer obtained because the temperature of the combustion catalyst is controlled only with great difficulty and the combustion catalyst is fused and the activity of the catalyst is sharply deteriorated.
In short, when a palladium system is used as a catalytically active component of the catalyst for the gas turbine combustor, accurate control of the fuel concentration in the combustion gas and the temperature and the flow rate of the combustion gas forms an indispensable condition. This fact constitutes a grave problem to be encountered in the feasibilization of the catalyst.